1. Field of the Invention
This invention relates to a process for the manufacture of multilayer printed circuit boards. In particular, this invention relates to a novel composition for treatment of copper and its alloys to form a surface having a uniformly etched and conversion coated surface with desirable properties for lamination of circuit layers in multilayer circuit fabrication.
2. Description of the Prior Art
Multilayer printed circuit boards are used for a variety of electrical applications and provide the advantage of conservation of weight and space. A multilayer board is comprised of two or more circuit layers, each circuit layer separated from another by one or more layers of dielectric material. Circuit layers are formed by applying a copper layer onto a polymeric substrate. Printed circuits are then formed on the copper layers by techniques well known to the art, for example print and etch to define and produce the circuit traces--i.e., discrete circuit lines in a desired circuit pattern. Once the circuit patterns are formed, a stack is formed comprising multiple circuit layers separated from each other by a dielectric layer, typically epoxy. Once the stack is formed, it is subjected to heat and pressure to form the laminated multilayer circuit board.
Following lamination, the multiple circuit layers are electrically connected to each other by drilling through holes through the board surface. Resin smear from through-hole drilling is removed under rather stringent conditions, for example, by treatment with a concentrated sulfuric acid or hot alkaline permanganate solution. Thereafter, the through-holes are further processed and plated to provide a conductive interconnecting surface.
Prior to lamination and through hole formation, the discrete copper circuit lines are typically treated with an adhesion promoter to improve bond strength between each circuit layer and adjacent interleaving dielectric resin layers. One method used by the art to improve bond strength involves oxidative treatment of the copper circuit lines to form a copper oxide surface coating on the circuit lines. The oxide coating is usually a black or brown oxide layer that adheres well to the copper. The oxide possesses significantly more texture or roughness than an untreated copper surface. Chemical treatments which produce adherent conversion coatings on metal surfaces, such as black oxide, are very commonly used as to promote adhesion of organic materials to metals. Other examples include metal phosphate coatings used as paint adhesion promoters. Such roughened and conversion coated surfaces enhance adhesion and wettability to the adjacent insulating layer by a mechanism that is believed to include mechanical interlocking between the metal surface and a dielectric resin layer. Metal surfaces that have been microetched, but not conversion coated do not generally possess as high a degree of surface roughness and texture, as can be inferred from their greater reflection of visible light.
Oxide layers are most often formed using highly alkaline solutions containing an oxidizing agent, typically sodium chlorite as disclosed in U.S. Pat. Nos. 2,932,599 and 4,844,981.
As described above, following formation of a multilayer stack, metallized through holes are formed to provide electrical connections between circuit layers. The formation of the metallized holes involves treatment with acidic materials. The acidic materials have a tendency to dissolve the copper oxide on the circuit lines where exposed in a through hole, interfering with bond between the circuit lines and the dielectric resin material and often causing a condition known in the art as pink ring. To reduce the susceptibility of the oxide to such attack, the oxide treatment described above is often followed by a step of converting the copper oxide to a form less soluble in acid while retaining enhanced surface roughness. Exemplary processes include reduction of the oxide by treatment with a reducing solution such as dimethylamine borane as shown in U.S. Pat. No. 4,462,161, an acid solution of selenium dioxide as shown in U.S. Pat. No. 4,717,439, or a sodium thiosulfate solution as shown in U.S. Pat. No. 5,492,595. An alternative approach involves partial or complete dissolution of the oxide layer to provide a copper surface having enhanced texture as shown in U.S. Pat. No. 5,106,454.
Other techniques known in the art to promote adhesion between copper surfaces and dielectric resins prior to multilayer lamination include the use of etches inclusive of cupric chloride etchants, mechanical treatments designed to produce surface texture, and metal plating, all designed to produce roughened surfaces. Historically, mechanical treatment and chemical etching procedures have not generally found wide acceptance in the industry, most likely due to deficiencies in both process consistency and in the bond strength to the dielectric material. Electrolytic metal plating processes may provide highly roughened surfaces and are commonly used to enhance adhesion of continuous sheets of copper to epoxy for formation of copper circuit board laminates. However, the innerlayers of a printed circuit board contain many electrically discrete circuit traces which prevent use of a process requiring electrical connection to all areas to be plated.
Oxidizing solutions containing peroxide are well known in the art. Such solutions have been used for a variety of purposes including removal of oxide scale, cleaning of surfaces, creation of smoother, brighter metallic surfaces and creation of microroughened metal surfaces. For example, in CA-A-1250406, metals such as iron, copper or their alloys are treated using a solution comprising hydrogen peroxide for metal pickling or polishing. The hydrogen peroxide solution contains a stabilizer, optionally a corrosion inhibitor such as benzotriazole, and an anionic or non-ionic surfactant. Because hydrogen peroxide decomposition is a problem, many hydrogen peroxide based compositions have been developed, each comprising a different type of stabilizing system.
Cleaning or polishing compositions based on hydrogen peroxide are described, for example, in U.S. Pat. No. 3,556,883 which discloses compositions comprising hydrogen peroxide, sulfuric acid and alcohol stabilizers for cleaning of, for example, metal wires. Other similar cleaning compositions are described in U.S. Pat. No. 3,756,957 where a stabilizer for hydrogen peroxide is selected from the group of aliphatic amines and their salts, alkoxy amines, aliphatic acid amines and aliphatic amines.
For use in the printed circuit industry, hydrogen peroxide etching solutions are known and have been described as etchant compositions for use in an etching step for forming a copper circuit pattern from copper laminate mounted on an insulating layer protected in a pattern corresponding to a final desired circuit pattern. The foil is then contacted with the etching solution and the unprotected copper leaving the desired circuit pattern. During the etching processes, the copper foil contacted with the hydrogen peroxide-based composition is etched away for complete removal. Peroxide etchants are described, for example, in U.S. Pat. Nos. 4,144,119; 4,437,931; 3,668,131; 4,849,124; 4,130,454; 4,859,281 and 3,773,577. In the latter two references, the etching composition also comprises a triazole to increase etch rate.
In U.K. Patent No. 2,203,387, a copper etching process is described with an etch bath regeneration step. A hydrogen peroxide etching composition comprising stabilizers including wetting agents is disclosed for cleaning copper surfaces of a printed circuit board prior to electroplating an additional copper layer onto the conducting layer which is formed from copper. After the electroplating step, a photoresist or screen resist is applied.
In U.S. Pat. No. 4,051,057, a bright dip composition for polishing/pickling metal surfaces, for instance of copper, comprises sulfuric acid, a hydroxy acid, such as citric acid, hydrogen peroxide, a triazole and/or a tertiary fatty-amine. The incorporation of surfactant is said to increase the rate of etching and oxide removal from the surface and the incorporation of benzotriazole is said to improve the "leveling effect".
In U.S. Pat. No. 3,948,703, chemical copper polishing compositions are described containing hydrogen peroxide, an acid and an azole compound. The compositions may also contain a surfactant and nonionic surfactants are used in the working examples.
In U.S. Pat. No. 4,956,035, chemical polishing compositions for metal surfaces comprise an etching composition, such as ferric chloride or peroxy sulfuric acid with a quaternary ammonium cationic surfactant and a secondary surfactant.
In U.K. Patent No. 2,106,086, hydrogen peroxide/acid compositions are used to etch, chemically mill or bright dip copper surfaces. The compositions contain triazole compounds to stabilize them against decomposition by heavy metal ions.
In Japanese published application No. 06-112,646, a copper surface is roughened to improve adhesion in laminates for the production of multilayer printed circuit boards. The roughening is carried out a by two-step process, each step involving treatment with a hydrogen peroxide/sulfuric acid composition. Both compositions must be free of a corrosion inhibitor.
In Japanese applications Nos. 03-140481 to 03-140484, copper surfaces are pretreated prior to lamination with a hydrogen peroxide/sulfuric acid composition to form a roughened surface. In 03-140484, the composition contains an additive (CB-596) manufactured by the Xekki Co., which is said to accelerate the process and inhibit decomposition of peroxide.
In U.S. Pat. No. 3,773,577, a copper etchant based on sulfuric acid and hydrogen peroxide contains an aliphatic amine, examples being primary or tertiary amines. The amines are not surface active. In Japanese application 03-79,778 a copper etchant based on sulfuric acid and hydrogen peroxide contains a triazole and chloride ion together with an alcohol or glycol. In Japanese application No. 51-27,819 a copper etchant based on hydrogen peroxide and sulfuric acid contains a tetrazole and optionally a tertiary amine or an alcohol.
In PCT application Publication No. WO 96/19097 published June 20, 1996, it is reported that a hydrogen peroxide containing aqueous composition used on a copper surface forms a cleaned copper surface which is both micro-roughened and conversion coated so as to have sufficiently good porosity that enables a strong bond to form with an organic layer as required in the preparation of multilayer boards. In accordance with the publication, it is disclosed that a process is provided for treating a metal surface comprising contacting the conducting layer of a circuit with an adhesion promotion composition comprising 0.1 to 20% by weight hydrogen peroxide, an inorganic acid, an organic corrosion inhibitor and a surfactant in an adhesion promotion step to form a micro-roughened conversion-coated surface. It is stated that the process is particularly useful for forming multilayer printed circuit boards comprising an innerlayer and an outerlayer, the innerlayer comprising at least one insulating layer and at least one conducting layer and the outerlayer comprising at least an insulating layer, in which the conducting layer is the metal surface treated with the peroxide composition. After the adhesion promotion, a polymeric material is adhered directly to the conducting layer of the innerlayer. The polymeric material may be the insulating layer of the outerlayer or for direct adhesion to the insulating layer of the outerlayer. It is further stated that the process may be used to provide a roughened surface to which polymeric materials such as photoimageable resins, solder masks, adhesives or polymeric etch resists have improved adhesion, usually in the manufacture of printed circuit boards. Finally, it is stated that the process is advantageous as it overcomes the need for forming a black or brown copper oxide layer, as in prior art processes.
Each aforesaid publication is incorporated herein by reference.
Though the process of the World Application identified above achieves many of the objectives stated, the presence of the surfactant may cause difficulties. Formulations containing surfactants are often difficult to use in the spray mode because of excessive foaming of the solution. Rinsing of the surfactant from the coating is often difficult because a surfactant often bonds to a surface. If surfactant is left on the surface, it may interfere with bond strength between the oxide coating and the surface adhered to the coating.